US2885313A - Process of treating magnesium-bearing aluminum base alloys with fluoroborate - Google Patents

Description

United States Patent Ofiice Patented May 5, 1959 PROCESS OF TREATING MAGNESIUM-BEARING lrZLXUMINUM BASE ALLOYS WITH FLUOROBO- Spencer R. Milliken, Lower Burrell Township, Westmoreland County, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application March 26, 1958 Serial No. 723,990

a 8 Claims. (Cl. 1486.27)

This invention relates to the thermal treatment of articles of aluminum base alloys containing magnesium. More particularly, it is directed to a treatment for aluminum-magnesium alloys to prevent subsequent atmospheric and high-temperature oxidation and corrosion, and to facilitate degassing of finished and semi-finished articles of such alloys.

This application is a continuation-in-part of my co- 18, 1957, now abandoned.

Thermal treatments, such as preheating for hot-working, annealing, solution heat-treatment and aging, are generally employed during the fabrication of products from aluminum and aluminumbase alloys. These treatments are usually carried out in air atmosphere furnaces. In an air atmosphere which has not been dried and at temperature above 800 F. butbelow the melting point of the alloy, aluminum-magnesium alloy articles tend to blister and stain, resulting in an objectionable appearance and sometimes a considerable decrease in mechanical strength. This effect is referred, to as high-temperature oxidation.

Some finished and semi-finished articles of aluminummagnesium alloys have been found to contain appreciable quantities of hydrogen, which may give rise to objectionable discontinuities in the metal structure. These discontinuities are generally quite easily detectable by ultrasonic testing techniques. It has been proposed to heat such articles in air for prolonged periods of time to diffuse hydrogen from the metal into the surrounding atmosphere. However, it has been found that the presence of small quantities of moisture in the surrounding atmosphere results in the failure of such degassing. This failure to degas may be explained by reaction between the metal surface and moisture forming metal oxides and a high hydrogen partial pressure at the metal surface which prevents loss of hydrogen from the metal into the surrounding atmosphere. In some cases, the hydrogen partial pressure may be sufficiently high to cause additional gassing of the metal.

To minimize high temperature oxidation, various methods have been employed to protect these alloys at temperatures above 800 E. US. Patent 2,092,033 to Stroup discloses the additionof fluorine-containing substances 1 in vapor form to a non-dried air atmosphere to provide a protective environment for such alloys. The vapor is conveniently generated from solid compounds which decompose at the elevated temperatures, above about 800 F. The metal is usually charged to a furnace already heated to the desired high temperature and containing the protective atmosphere or supplied with the vaporgenerating fluoride compounds if the protective atmosphere has not been previously provided, and it is quickly l heated to the desired temperature.

US. Patent 2,379,467 to Abbe discloses a protective treatment for aluminum alloy forgings in which an aqueous solution of sodium fluoroborate is applied to the forging which is then dried prior to being heated to the pending application, Serial No. 684,642, filed September solution heat treatingtemperature. The coated forging is quickly heated to the required temperature in accordance with conventional practice.

Although these processes have been helpful in many cases, they have not eliminated altogether the undesirable blistering and staining which often occurs on articles of aluminum-magnesium alloys, nor have they proven effective in reducing the gas content of the metal.

It has been found that the blistering and staining of such alloys is apparently related to the oxidation of magnesium atoms at the external surface of the article and that the particles of these oxides or other oxy-compounds act as focal points or channels for further oxidation on and within the surface. This oxidation is apparently caused by the attack of water upon the surface, oxidizing the aluminum and magnesium and releasing hy drogen.

It has further been determined that above a certain criticaltemperature, there is a rapid increase in the rate of magnesium atom diffusion to the surface and concomitant oxidation of the magnesium. Whereas in previous methods oxidation has been inhibited by providing a protective atmosphere above that critical temperature, it has now been discovered that, for complete protection of aluminum-magnesium alloys, they must be subjected to preliminary vapor treatment below that temperature, more particularly, at a point :below the onset of any substantial amount of magnesium oxidation.

It is an object of this invention to provide a preliminary thermal treatment for aluminum-magnesium alloy articles which substantially prevents high-temperature oxidation.

Another object is to provide a treatment for aluminummagnesium alloy articles which will promote subsequent degassing of such articles in an atmosphere containing moisture.

It is also an object to provide a treatment for such articles which develops a surface condition that permits storage in industrial atmospheres for prolonged periods of time without corrosion.

A further object is to provide surface protection on aluminum-magnesium alloy articles which is stable at elevated temperatures in the presence of oxygen and moisture.

It has been found that the foregoing objects can be attained by a method of treating finished or semifinished articles of aluminum "base alloys containing magnesium wherein the articles are coated with an organic ammonium fiuoroborate which yields boron trifluoride when heated to a temperature between 400 F. and 750 F., and heating the coated article to a temperature between 400 F. and 750 F., but in all cases below the critical oxidation temperature of the alloy, for a period of time sufficiently long to cause substantial disappearance of the organic substance whereby a protective film is developed on the surface of the article which is substantially free from organic material. The alloy articles which have been treated in this manner strongly resist oxidation at temperatures above 800 F. and up to the melting point of the lowest melting point constituent in the alloy which is often referred to as the temperature of incipient fusion. In addition the treated articles may be degassed by subsequent heating for prolonged periods of time, even in the presence of moisture.

The critical oxidation temperature of an aluminummagnesium alloy is defined as the lowest temperature at which the formation of magnesium oxide can be detected by electron diffraction techniques. This may be a particular temperature or a very narrow temperature range of the order of 20 F., and hence the term is intended to cover both conditions. It is directly dependent upon the humidity of the furnace atmosphere, and the alloy composition, primarily the magnesium content. Although it can be determined readily for each set of conditions, it has been observed that these points fall within the range of 575 to 750 F., and generally 600 to 675 F.

- Generally, the electron diffraction determination involves heating samples in air to various temperatures and under various conditions of humidity and time. After such treatment, the samples are exposed to an electron beam in diffraction apparatus wherein the beam is refiected from the surface of the samples and a pattern ismade. upon a film as in X-ray diffraction studies. By comparing the pattern with that of a known sample, the nature of the substances on the surface of the metal sample can be determined. This diffraction pattern provides. a qualitative indication of the presence of magnesium oxide.

The term. aluminum-magnesium alloy as herein employed refers to an aluminum base alloy containing on a weight basis 0.1 to percent magnesium, with or without the presence of other elements, such as from 0.1 to 12 percent copper or 0.25 to 14 percent silicon or 0.1 to. percent zinc or 0.1 to 3 percent manganese, or combinations of two or more of these elements. Any of the foregoing alloys may also contain one or more of the following elements, often referred to as hardeners, in the following percentages:

0.05 to 0.5 percent chromium 0.01 to 0.5 percent titanium 0.25 to 2.5 percent nickel 0.01 to 0.5 percent boron 0.002 to 2 percent beryllium 0.1 to 0.5 percent molybdenum 0.1 to 0.5 percent zirconium 0.1 to 0.5 percent tantalum 0.1 to 0.5 percent colombium 0.1 to 0.5 percent cobalt However, the total amount of the latter elements should not exceed about 3 percent.

The term aluminum base alloy as used herein refers to those compositions which contain at least 50 percent by weight of aluminum.

The term organic boron trifiuoride-carrier compound as used herein refers to organic ammonium fluoroborates which yield boron trifiuoride in a reactive state between 400 and 750 F. These compounds'may be described as the products of reaction between fluoroboric acid, HBF and the constituent amine or amide. Generally, it is preferred to use compounds which decompose or yield boron trifiuoride above 400 F. but below 575 F., the lowest critical oxidation temperature observed for an aluminum-magnesium alloy. The boron trifiuoride, or substance containing boron trifiuoride in a reactive state, is generally produced by the pyrolytic decomposition of. the carrier compound. Compounds of this group may beused singly or in combination.

Examples of such carrier compoundsare the. aliphatic ammonium fluoroborates, di-n-butyl ammonium tetrafiuoroborate, n-octadecylN,N,N, trimethyl tetrafiuoro borate, and di-n-amyl ammonium tetrafiuoroborate.

It has been found that the boron trifiuoride-carrier compounds should yield substantially no boron trifiuoride below about 400 F. since the protective film, if any, developed below this temperature is considerably less effective as an inhibitor than that produced by boron trifiuoride above 400 F., possibly due to the formation of a different crystalline structure in the surface reaction product. Nevertheless, the carrier compound may decompose over a wide temperature range above 400 F. as is generally experienced in commercial products of this type which contain. impurities. However, some decomposition may commence below 400 F., but the amount is small and can be disregarded.

The organic boron trifiuoride-carrier compound is preferably applied to the surface of the aluminum-magnesium alloy in a solution, although it may be applied in a molten or liquid state. The solution may be applied by spraying, swabbing, or immersing the article. The preferred embodiment of this invention utilizes an alcoholic or aqueous solution of the organic boron trifiuoridecarrier compound containing at least 0.4 percent by Weight of boron trifiuoride and preferably about 2.6 percent. In addition, wetting or other surface active agents may be added to improve the application.

The coated articles are heated to a temperature of 400 F. or above where they are maintained until the organic substance substantially disappears and the fluoride film, which is substantially free from organic material, is formed. Periods from one to forty-five minutes have been satisfactory to accomplish this purpose. Treatment periods of less than one minute do not permit development of a satisfactory film or substantial disappearance of the organic coating while treatments in excess of forty-five minutes do not provide any additional protection' or'offer any further benefit. Generally, periods from one to twenty minutes are sufiicient; however, longer times of treatment are desirable in the case of alloys particularly susceptible to oxidation or when the surface oxide film is of greater thickness than that normally developed by exposure to the atmosphere.

The alloy articles treated in accordance with this in vention may be subjected immediately to higher heattreating temperatures or may be removed for storage or subsequent heat treatment. The protective film developed effectively prevents atmospheric corrosion by industrial fume.

The present process may be practiced in an untreated air atmosphere, i.e., a normal air atmosphere as commonly employed in heat-treating furnaces. No drying of the air need be undertaken as moisture can be tolerated; in fact, the invention has been successfully employed even in furnace atmospheres having dew points on the order of F. Also, observations indicate that more satisfactory results are obtained when the furnace atmosphere contains at least grains of water per cubic foot of furnace volume, but it should not exceed about 45 grains per cubic foot. In addition, the atmosphere may be contaminated with such substances as sulfur dioxide, hydrogen chloride, ammonia, carbon monoxide and carbon dioxide Without adverse effect upon the action of the boron trifiuoride. I

Gases which are inert toward the aluminum-magnesiumalloys may be employed in place of air such as nitrogen, argon, helium andfuel gas. The term atmosphere as used herein includes air, the inert gases, or combinations thereof.

Further, this invention is most effectively practiced when the alloy articles have not previously been subjected to temperatures above their critical oxidation temperature. Thepresence of magnesium oxide on the surface, which has been produced. in preceding thermal treatments, is observed to reduce the effectiveness of boron trifiuoride, presumably due to the formation of. oxy-fiuoride complexes, but there is still substantial benefit to be derived from the treatment of the present invention.

The temperature employed for the subsequent degassing treatment should be above 750 F. but below the temperature of incipient fusion of the alloy. As is well known, the higher the temperature, the greater the rate of diffusion and the shorter the time required. Generally, for aluminum-magnesium alloys a temperature of about 900 to 975 F. has been found satisfactory.

The period for the degassing treatment will depend primarily on the thickness of the article being treated (the shortest diffusing path). Generally at 940 F. periods in excess of several hours and up to 20 days or more are necessary; for articles having a maximum crosssection of /2 inch, periods in excess of 16 hours have been found adequate. A thickness of 1 inch will generally require a period in excess of'40 hours at the same temperature; to days may be required for crosssections, i.e. thicknesses, larger than 3 to 4 inches. Generally, the time required for the treatment will be related to the rate of hydrogen difiusion in the alloy at a given temperature.

Subsequent to the degassing treatment, the treated article should be subjected to a working step to effect some reduction in size, thus healing the voids left by the diffused hydrogen. The percentage of reduction necessary will be determined by the nature of the article and its original gaseous content.

The term degassing, as used herein, contemplates both the removal of gas from the metal and the prevention of regassing of the metal.

The etficacy of the present invention is illustrated by the following examples:

Example 1 A lot of 0.01 inch foil test strips of an alloy consisting of aluminum, 1.5 percent magnesium, 4.5 percent copper and 0.65 percent manganese was divided into two groups of specimens and heat treated for 16 hours at 920 F. in an air atmosphere having a dew point of 125 F. The first group of specimens had been pretreated by swabbing with a solution containing 7.5 percent by weight of din-butyl ammonium tetrafluoroborate in isopropyl alcohol and heated to a temperature of 500 to 550 F. for a period of five minutes. Upon removal from the heat-treating operation, the first group of specimens was found to be free from blistering and staining but the untreated group was moderately blistered and had a grey-black stain.

Example 2 A lot of 0.01 inch foil test strips of. an alloy consisting of aluminum, 2.5 percent magnesium and 0.25 percent chromium was divided into two groups of specimens and heated 16 hours at a temperature of 950 F. in an air atmosphere having a dew point of 125 F. The first group of specimens had been pretreated by swabbing with a solution containing 10 percent by weight of n-octadecyl- N,N,N, trimethyl ammonium tetrafluoroborate in isopropyl alcohol and heated to a temperature of 550 F. for a period of ten minutes. Upon examination the pretreated group of specimens was found to be free from blistering and staining while the untreated was moderately blistered and had a dark grey stain.

Example 3 A lot of 0.01 inch foil test strips of an alloy consisting of aluminum, 1.0 percent magnesium and 0.3 percent manganese was divided into two groups of specimens and heated for 16 hours at a temperature of 950 F. in an air atmosphere having a dew point of 125 F. The first group of specimens had been pretreated by swabbing with a solution containing 7 percent by weight of di-namyl ammonium tetrafluoroborate in isopropyl alcohol and heated to a temperature of 575 F. for a few minutes. Upon inspection the pretreated group was free from blistering and staining but the untreated was moderately blistered and had a grey-black stain.

Example 4 A lot of sheet ingots of an alloy consisting of aluminum, 2.5 percent magnesium, and 0.25 percent chromium was divided into two groups of specimens and preheated for 30 hours at a temperature of 950 F. prior to hot-rolling into sheet. The first group had been pretreated by swabbing with a solution of 7.5 percent by weight of di-n-butyl ammonium tetrafluoroborate in isopropyl alcohol and heated through a temperature range of 450 to 575 F. for a period of thirty minutes. Upon inspection after preheating, the pretreated ingots were found to be free from staining although the untreated had a greyblack to blue-black stain indicating heavy'oxidation.

Example 5 A lot of /2 inch thick specimens was taken from a forging ingot of an alloy nominally consisting of aluminum, 4.4 percent copper, 0.9 percent silicon, 0.8 percent manganese and 0.4 percent magnesium. The ingot had been rejected because of porosity. The specimens were divided into two' groups and heated at 950 F. for 16 hours in an air atmosphere having a dew point of about F. One group of specimens was pretreated by swabbing with a solution containing 7% percent by weight of di-n-butylammonium tetrafluoroborate in isopropyl alcohol and heated at a temperature of 450 to 550 F. for about 15 minutes. The other group received no preliminary treatment. Upon removal from the furnace, both groups of specimens were subjected to gas evaluation by rapidly heating the specimens under reduced pressure to magnify voids. Upon inspection, the treated group of specimens wasfound to be free from such magnified voids whereas the untreated showedmoderate to considerable quantities of such magnified voids. This test indicates that the treatment of the present invention enabled the degassing of even rejected metal by heating for extended periods of time in an atmosphere containing moisture.

Having thus described my invention, I claim:

1. The method of treating finished or semi-finished articles composed of aluminum base alloy containing from 0.1 to 15 percent magnesium comprising the steps of coating said articles with an organic ammonium fluoroborate which yields BF when heated to a temperature above 400 F. but below 750 F., and heating said coated article to a temperature between 400 and 750 F., but in all cases below the critical oxidation temperature of said alloy, for a period of time sufliciently long to cause substantial disappearance of the organic substance.

2. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent magnesium comprising the steps of coating said articles with an organic substance selected from the group consisting of di-n-butyl ammonium tetrafluoroborate, n-octadecyl-N,N,N, trimethyl ammonium tetrafluoroborate, and di-n-amyl ammonium tetrafluoroborate, and heating said coated article to a temperature between 400 and 750 F., but in all cases below the critical oxidation temperature of said alloy, for a period of time sufiiciently long to cause substantial disappearance of the organic substance.

3. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent magnesium comprising the steps of coating said articles with an organic ammonium fluoroborate which yields BF when heated to a temperature above 400 F. but below 575 F., and heating said coated article to a temperature between 400 and 575 F. for a period of time sufliciently long to cause substantial disappearance of the organic substance.

4. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent magnesium comprising the steps of coating said articles with an organic substance selected from the group consisting of di-n-butyl ammonium tetrafluoroborate, n-octadecyl-N,N,N, trimethyl ammonium tetrafluoroborate, and di-n-amyl ammonium tetrafluoroborate, and heating said coated article to a temperature between 400 and 575 F. for a period of time sufiiciently long to cause substantial disappearance of the organic substance.

5. The method of treating finished or semi-finished articles composed of an aluminum 'base alloy containing from 0.1 to 15 percent magnesium comprising the steps of coating said articles with an organic ammonium fluoroborate which yields BF; when heated to a temperature above 400 F. but below 750 F., and heating said coated article in an air atmosphere to a tempera- 7 ture between 400 and 750?- F., but in all cases below the critical, oxidation temperature of said alloy, for a period of time sufficiently long to cause substantial disappearance of the organic substance.

6. The method in accordance with claim 5 wherein the air atmosphere contains at least but not more than 45 grams of water per cubic foot;

7. The method of treating finished or semi-finished articles composed of an aluminum base alloy containing from 0.1 to 15 percent magnesium to improve the resistance to corrosion at room temperature and resistance to oxidation at elevated temperatures, said method comprising the steps of coating said articles with an organic ammonium fluoroborates which yields BF when heated to a temperature above 400 F. but below 750 F., and heating said coated article to a temperature between 400 and 750 F;, but in all cases below the critical oxidation temperature. of said alloy, for a period of time sufiiciently long to cause substantial disappearance oil the organic; substance whereby a. protective film is developed onthe. surface: of said article that is substantially free. fromv organic material.

8. The method of treating finished or semi-finished articles composed. of. an aluminum base alloy containing from 0.1 to 15 percent magnesium prior to degassing by thermal treatment at temperatures above 750 F. but belowthe temperature of incipient fusion comprising the steps of coating said articles with an organic ammoniumfluoroborate which yields BF when heated to a temperature above 400 F. bntbelow 750 F., and heating said coated article to a temperature between 400 and 750 F., but in all cases below the critical oxidation temperature of saidalloy, for a period of time sufficient- 1y long to cause substantial disappearance of the organic substance.

No references cited.

Claims (1)

1. THE METHOD OF TREATING FINISHING OR SEMI-FINISHED ARTICLES COMPOSED OF ALUMINUM BASE ALLOY CONTAINING FROM 0.1 TO 15 PERCENT MAGNESIUM COMPRISING THE STEPS OF COATING SAID ARTICLES WITH AN ORGANIC AMMONIUM FLUOROBORATE WHICH YIELDS BF3 WHEN HEATED TO A TEMPERATURE ABOVE 400*F, BUT BELOW 750*F., AND HEATING SAID COATED ARTICLE TO A TEMPERATURE BETWEEN 400 AND 750*F., BUT IN ALL CASE BELOW THE CRITICAL OXIDATION TEMPERATURE OF SAID ALLOY, FOR A PERIOD OF TIME SUFFICEINTLY LONG TO CAUSE SUBSTANTIAL DISAPPEARANCE OF THE ORGANIC SUBSTANCE.